Crystallization of columbite-tantalite in pegmatites: Experimental constraints and implication for natural systems

钽(Ta)是高科技产业的关键元素，经济上的含Ta矿物主要集中在花岗伟晶岩中，主要是柱状钽矿群的矿物[(Mn，Fe)(Nb，Ta)206]-我们对经济的Ta矿床形成的理解需要有关含Ta矿物的结晶条件的信息，如结晶温度和Ta在伟晶熔体中的溶解度，这是目前尚不能获得的信息。伟晶系统中的结晶过程可在900至400℃的宽温度范围内发生，对结晶温度的限制对于模拟导致经济浓度的含Ta矿物形成的反应至关重要。正在进行的一项提议的目标是获得关于铀矿组矿物结晶条件的实验数据。该项目围绕两点进行：i)在高熔点伟晶熔体中测量作为温度函数的锂族矿物的锰端元的溶解度；ii)校准温度计，以限制含Ta量矿物(锆石中的Ta和钽矿中的Zr；锰钨矿和铁辉橄榄岩对的稳定场)的结晶温度范围。这些实验是在研究天然伟晶岩中与Ta矿化有关的锆石的地球化学和结构特征的同时进行的。在过去的14个月里，已经进行了不同类型的实验，并取得了成功。现有的结果是非常有希望的，应该会对我们对导致钽矿化的过程的知识产生强烈的影响。为了成功地完成上述任务(一)和(二)，有必要进行补充性研究，并证明将该项目延长几个月是合理的。在此延长期间，我们将重点关注三点：i)我们将更详细地测定锰钽矿和锆石的溶解度对熔体ASI[Al/Na+K+Li]的依赖，以表征过量铝和过量碱对硅酸盐熔体中稀有金属阳离子络合的作用；2)我们将在低温下校准锆钽矿温度计；以及3)我们将研究Hf和Ta在锆石中的掺杂机制，因为与钽矿伴生的天然锆石总是含有大量的Hf。

Tantalum (Ta) is a key element of the high-tech industry and economic concentrations of Tabearing minerals occur exclusively in granitic pegmatites, mainly as minerals from the columbitetantalite group [(Mn,Fe)(Nb,Ta)206]- Our understanding of the formation of economic Ta ore deposits requires information on the crystallization conditions of Ta-bearing minerals, such as crystallization temperature and Ta solubility in pegmatitic melts, which is not available up to now. Crystallization processes ih pegmatitic systems can occur over a wide temperature range from 900 to 400°C and constraints on crystallization temperatures are crucial to model the reactions leading to the formation of Ta-bearing minerals in economic concentrations. The goal of an ongoing proposal was to get experimental data on the crystallization conditions of columbite-group minerals. The project was built around two points: i) solubility measurements of the manganese end members of the columbite-group minerals as a function of temperature in highly-fluxed pegmatitic melts and ii) the calibration of thermometers to constrain the temperature range of crystallization of Tabearing minerals (Ta in zircon and Zr in tantalite; stability field of manganotantalite and ferrotapiolite pairs). The experiments were conducted in parallel with investigations of the geochemistry and textural features of zircon associated with Ta mineralization in natural pegmatites. Different types of experiments have been conducted with success in the last 14 months. The available results are very promising and should have a strong impact on our knowledge of processes leading to Ta mineralizations. To complement with success the tasks (i) and (ii) listed above, complementary studies are necessary and justify the prolongation of the project for a few months. During this prolongation, we will focus on 3 points: I) we will determine in more detail the solubility dependence of manganotantalite and zircon on the ASI [Al/Na+K+Li)] of the melt, in order to characterize the role of excess Al and excess alkalis on the complexing of rare metal cations in silicate melts; 2) we will calibrate the Zr-in-tantalite thermometer down lo low temperatures; and 3) we will investigate the incorporation mechanisms of hafnium and tantalum in zircon because natural zircon associated with tantalite always coniain significant amounts of Hf.